Oil and gas pipelines require routine maintenance. A substantial part of this maintenance involves the deployment of pipeline “pigs”—typically bullet-shaped foam or flexible urethane devices—that pass through the pipeline while pressing against the pipeline wall. The pigs can remove both solid and liquid buildup from the pipeline to increase the flow and the efficiency of the pipeline, and to help in controlling and monitoring corrosion.
However, a continuing need exists for faster, more accurate, and more cost effective devices and systems for locating and tracking pigs used in pipelines. While traveling through a pipeline, a pig will sometimes become stuck. The consequences of a pipeline pig becoming stuck in a pipeline due to, for example, a dent in the pipeline, an out-of-round shape of a segment of the pipeline, a partially closed in-line valve, or other reasons can be difficult and costly to address.
There are a variety of prior art methods used for tracking pigs within pipelines. One method involves attaching an electromagnetic transmitter to the pig and then using an electromagnetic receiver on the outside of the pipeline to detect the electromagnetic signal and locate the pig. Another approach has involved attaching an acoustic pinger to the pig and then using an acoustic receiver on the outside of the pipeline to detect and locate the source of the acoustic signal.
Acoustic pipeline pig tracking is used in offshore, subsea, and other underwater pipelines. This is because the relatively low frequency and low energy signals generated by an acoustic pinger do not propagate well within a gaseous media. Because of this limitation, acoustic pig tracking systems require that the pipeline be located within a liquid medium (e.g., sea or lake water) and typically further require that the pipeline also be filling with a liquid medium (e.g., crude oil, refined liquid hydrocarbon products, etc.).
While acoustic pig tracking systems are only useful for underwater tracking, they have distinct advantages for use in such applications. Foremost among these advantages is range. While electromagnetic tracking systems typically have a range of only a few tens of meters, acoustic pig tracking systems can detect the presence of an underwater pipeline pig from several kilometers away. This advantage would be significant if it could be utilized to reduce or eliminate the need to deploy divers or remote operated vehicles (ROVs) to monitor the progress and location of the pig.
Heretofore, the acoustic pig tracking transmitters carried by underwater pipeline pigs have transmitted a ping signal which has consisted of only a single frequency tone. This tone varies by manufacturer and pinger but is typically in the range of from 8 kHz to 40 kHz. An acoustic receiver is used shipside to listen for this single tone and to downshift the signal into the range of human hearing. This type of acoustic pinger can be likened to ringing a bell wherein the single ping tone is emitted repeatedly.
One significant shortcoming of the prior art single tone transmitter and system is that the signal can be severely degraded, distorted, or even lost entirely due to multipathing. Multipathing is produced by the tone being reflected in the underwater environment by (a) the sea bottom, (b) the water surface, (c) thermoclines, and (d) other underwater surfaces presented, for example, by natural topographic formations, work vessels, oil platforms, etc. In addition, noise from the environment and from work vessels and ROVs can overlap with and obscure or cancel out the frequency of interest without warning.
The multipathing behavior of a single tone underwater signal emitted from a prior art transmitter 2 installed in a pipeline pig 4 is illustrated in FIG. 1. The pig 4 is located within a liquid pipeline 6. The acoustic pinger 2 within the pipeline 6 transmits acoustic energy into the surrounding water. Some portion of the acoustic energy is reflected by thermoclines 8 and 10 in the water. Additionally, subsea structures 12 and 14 reflect the signal as well.
Each of these reflections splits the acoustic energy and ultimately delays the arrival of some portion of the energy at the receiver (i.e., a hydrophone) 16 carried by the surface vessel 18. Moreover, as the energy arrives from the multipath sources, its phasing has an additive and subtractive effect on the total amount of acoustic energy received by the hydrophone.
Consequently, due to the effect of multipathing, it is possible to be quite near the acoustic source 2 and yet not receive a reliable signal, or to detect a signal which seems to “come and go”. This significantly reduces the trustworthiness and reliability of the system. Also, another deficiency of the single tone systems is that very little information may conveyed by the single frequency, and the reliability of the information received is no better than the reliability of the single tone by which it is carried.
Therefore, a need exists for an improved acoustic transmitter and system for locating and tracking pipeline pigs used in underwater pipelines. The improved acoustic transmitter and system will preferably (a) significantly reduce or eliminate the effects of multipathing, (b) provide highly accurate and rapid location and tracking of pigs in underwater pipelines, (c) significantly reduce or eliminate the need to use divers and/or remote operated vehicles (ROVs) for pig locating and tracking operations, and (d) provide the ability to transmit significantly more information regarding the status or condition of the pig and the pipeline. In addition, the improved acoustic transmitter and system will preferably also be capable of (1) determining the relative speed of a moving pipeline pig, (2) accurately determining the estimated point location of the pipeline pig rather than simply determining that the pipeline pig and transmitter are within a sphere of detection, and (3) continuously tracking a moving pig from a moving surface vessel, without the need for divers and ROVs.